(a) Field of the Invention
The present invention relates to an in-plane-switching-mode liquid crystal display (IPS LCD) device and, more particularly, to the improvement of an IPS LCD device to achieve a wider viewing angle.
(b) Description of the Related Art
An IPS LCD device has the advantage of a wide viewing angle due to the configuration wherein the twisting direction of the liquid crystal (LC) is parallel to the surface of the substrates. The IPS LCD device generally includes an LC layer, a pair of glass substrates sandwiching therebetween the LC layer, and a pair of polarizing films sandwiching therebetween the glass substrates and the LC layer together. In the IPS LCD device, the LC layer is applied with a lateral electric field, which is parallel to the substrates, to control the direction of the LC molecules for image display.
It is known in the IPS LCD device that a chromaticity shift occurs, especially if the LCD device is observed in an angular direction of 45 degrees away from the polarization direction of the polarizing films. The chromaticity shift degrades the image quality of the LCD device. Patent Publication JP-A-10(1998)-307291 and -2001-242462, for example, describe techniques for suppressing the chromaticity shift in the IPS LCD device.
FIG. 19 shows a portion of the IPS LCD device described in JP-A-10-307291 in a sectional view, wherein a TFT substrate 211 and an associated polarizing film 219 on the TFT-substrate side are depicted, The polarizing film 219 includes a PVA (polyvinyl-alcohol) polarization layer 219C, a pair of triacetyl-cellulose (TAC) protective layers 219B and 219C sandwiching therebetween the PVA polarization layer 219D, and an optical compensation layer 219A disposed between the TAC protective layer 219B and the TFT substrate 211.
The TAC layer 219B has a negative retardation, whereas the optical compensation layer 219A has a positive retardation having a value equivalent to the absolute value of the negative retardation of the TAC layer 219B. It is recited in this publication that the optical compensation layer 219A compensates the retardation generated in the TAC layer 219B, thereby reducing the polarization component caused by the retardation in the TAC layer 219B to suppress the chromaticity shift.
In the structure described in the JP-A-10-307291, however, the polarization of the light incident onto the light-emitting-side polarizing film changes due to the retardation of the LC layer and the light dispersion caused by the color filters. Thus, the suppression of the chromaticity shift by reducing the leakage light in the above configuration is insufficient.
FIG. 20 schematically shows the IPS LCD device described in JP-A-2001-242462 in an exploded perspective view thereof. The LCD device includes two-axial retardation films 302 and 303 each disposed between the LC layer 301 and a corresponding one of the pair of polarizing films 304 and 305. The retardations of the retardation films 302 and 303 are substantially equal to the retardation of the LC layer 301. It is recited in this publication that the retardation films 302 and 303 should be preferably disposed so that the angles between the polarization axis of the light-incident-side polarizing film 304 and the slow axes of the retardation films 302 and 303 assumes zero to 30 degrees.
In JP-A-2001-242462, it is recited that a reversed transmittance phenomenon is suppressed by the configuration. The reversed transmittance is such that the direction of the change in the transmittance of the LC layer is reversed to the direction of the change in the driving voltage. In general, it is known in the IPS LCD device that the reversed transmittance phenomenon is observed in the wavelength range of green or blue when the driving voltage is changed for the LC layer in the range of intermediate gray scale levels. It is also recited in the publication that the chromaticity shift caused by the change of the viewing angle is suppressed by the configuration.
In the LCD device described in JP-A-2001-242462, however, it is not considered there is leakage light caused by the deviation between the polarization axis of the polarizing film 304 and the slow axes of the retardation films 302 and 303, as well as the leakage light caused by the deviation between the polarization axis of the polarizing film 304 and the optical axis of the LC layer 301. In general, the polarization axis of the polarizing film 304 deviates from the slow axes of the retardation films 302 and 303 by an angle of zero to 30 degrees. This deviation generates leakage light upon display of black to degrade the contrast ratio and thus degrade the image quality of the LCD device.
In view of the above problems in the conventional techniques, it is an object of the present invention to provide an IPS LCD device, which is capable of reducing the leakage light leaking upon display of black color as observed in the slanted viewing direction to thereby improve the contrast ratio of the IPS LCD device, and capable of suppressing the chromaticity shift between the normal viewing angle and a slanted viewing angle to thereby improve the image quality of the LCD device.
The present invention provides, in a first aspect thereof, an IPS LCD device including: a liquid crystal (LC) layer having a first optical axis; first and second substrates sandwiching therebetween the LC layer, the first and second substrates being disposed on a light-emitting side and a light-incident side, respectively, of the LCD device; first and second polarizing films sandwiching therebetween the first and second substrates and the LC layer, the first and second polarizing films having polarization axes extending normal to each other and being disposed on the light-emitting side and the light-incident side, respectively, of the LCD device, the second polarizing film including a first protective layer, a polarization layer and a second protective layer, which are disposed consecutively as viewed from the light-incident side, each of the first and second protective layers having a retardation depending on a thickness of the each of the first and second protective layer; and an optical compensation layer disposed between the first polarizing film and the second polarizing film, wherein: the optical compensation layer has a birefringence satisfying the relationship (ns−nz)/(ns−nf)<≦0.5, where ns, nf and nz are a refractive index along an in-plane slow axis, a refractive index along an in-plane fast axis, and a refractive index along a thickness direction, respectively, of the optical compensation layer; an angle between the in-plane slow axis of the optical compensation layer and the first optical axis is within ±2 degrees; and the optical compensation layer has an in-plane retardation N1 (nm), the second optical protective layer has a thickness D1 (μm) satisfying the following relationship:83.050−0.810×D1≦N1≦228.090−0.74×D1,in a range of 0<D1≦80.
The present invention also provides, in a second aspect thereof, an IPS LCD device including; a liquid crystal (LC) layer having a first optical axis; first and second substrates sandwiching therebetween the LC layer, the first and second substrates being disposed on a light-emitting side and a light-incident side, respectively, of the LCD device; first and second polarizing films sandwiching therebetween the first and second substrates and the LC layer, the first and second polarizing films having polarization axes extending normal to each other and being disposed on the light-emitting side and the light-incident side, respectively, of the LCD device; and first and second optical compensation layers disposed between the first polarizing film and the first substrate and between the second polarizing film and the second substrate, respectively, wherein: each of the first and second optical compensation layers has a birefringence satisfying the relationship (ns−nz)/(ns−nf)≦0.5, where ns, nf and nz are a refractive index along an in-plane slow axis, a refractive index along an in-plane fast axis, and a refractive index along a thickness direction, respectively, of the each of the first and second optical compensation layers; an angle between the in-plane slow axis of the first optical compensation layer and the first optical axis is within ±2 degrees, and an angle between the in-plane slow axis of the second optical compensation layer and the first optical axis is within 90±2 degrees; and the first and second optical compensation layers have in-plane retardations N1 (nm) and N2 (nm), respectively, satisfying the following relationship:29.87+1.79N2−0.048N22+0.001N23≦N1≦187.22−1.66N2+0.0475N22−0.0009N23 in a range of 0.6<N2≦46.
The present invention also provides, in a third aspect thereof, an IPS LCD device including: a liquid crystal (LC) layer having a first optical axis; first and second substrates sandwiching therebetween the LC layer, the first and second substrates being disposed on a light-emitting side and a light-incident side, respectively, of the LCD device; first and second polarizing films sandwiching therebetween the first and second substrates and the LC layer, the first and second polarizing films having polarization axes extending normal to each other and being disposed on the light-emitting side and the light-incident side, respectively, of the LCD device; and first and second optical compensation layers disposed between the first polarizing film and the first substrate and between the second polarizing film and the second substrate, respectively, wherein: each of the first and second optical compensation layers has a birefringence satisfying the relationship (ns−nz)/(ns−nf)≦0.5, where ns, nf and nz are a refractive index along an in-plane slow axis, a refractive index along an in-plane fast axis, and a refractive index along a thickness direction, respectively, of the each of the first and second optical compensation layers; an angle between the in-plane slow axis of each of the first and second optical compensation layers and the first optical axis is within ±2 degrees; and the first and second optical compensation layers have in-plane retardations N1 (nm) and N2 (nm), respectively, satisfying the following relationship:162.560−8.874N2+2.258N22−0.291N23+0.0165N24−0.000346N25≦N1≦142.465+2.546N2−0.017N22 in a range of 0.6≦N2≦22;73.04−0.977N2+0.0220N22≦N1≦142.465+2.546N2−0.017N22 in a range of 22<N2≦62; and73.04−0.977N2+0.00220N22≦N1≦1205.596−41.304N2+0.586N22−0.0028N23 in a range of 62<N2≦92:
The present invention also provides, in a fourth aspect thereof, an IPS LCD device including: a liquid crystal (LC) layer having a first optical axis; first and second substrates sandwiching therebetween the LC layer, the first and second substrates being disposed on a light-emitting side and a light-incident side, respectively, of the LCD device; first and second polarizing films sandwiching therebetween the first and second substrates and the LC layer, the first and second polarizing films having polarization axes extending normal to each other and being disposed on the light-emitting side and the light-incident side, respectively, of the LCD device; and first and second optical compensation layers disposed between the first polarizing film and the first substrate and between the second polarizing film and the second substrate, respectively, wherein: the first optical compensation layer has a birefringence satisfying the relationship (ns1−nz1)/(ns1−nf1)≦0.5, where ns1, nf1, nz1 are a refractive index along an in-plane slow axis, a refractive index along an in-plane fast axis, and a refractive index along a thickness direction, respectively, of the first optical compensation layer; the second optical compensation layer has a birefringence satisfying the relationship (ns2−nz2)/(ns2−nf2)≦−2, where ns2, nf2, nz2 are a refractive index along an in-plane slow axis, a refractive index along an in-plane fast axis, and a refractive index along a thickness direction, respectively, of the second optical compensation layer; an angle between the in-plane slow axis of the first optical compensation layer and the first optical axis is within ±2 degrees, and the second optical compensation layer has an optical axis substantially normal to a surface of second substrate; and the first and second optical compensation layers have in-plane retardations N1 (nm) and N2 (nm), respectively, satisfying the following relationship;36.859+7.617N2≦N1≦168.193+9.783N2 in a range of 0<N2≦6.0.
The present invention also provides, in a fifth aspect thereof, an IPS LCD device including: a liquid crystal (LC) layer having a first optical axis; first and second substrates sandwiching therebetween the LC layer, the first and second substrates being disposed on a light-emitting side and a light-incident side, respectively, of the LCD device; first and second polarizing films sandwiching therebetween the first and second substrates and the LC layer, the first and second polarizing films having polarization axes extending normal to each other and being disposed on the light-emitting side and the light-incident side, respectively, of the LCD device, each of the first and second polarizing films including a first protective layer, a polarization layer and a second protective layer, which are disposed consecutively as viewed from the light-incident side, each of the first and second protective layers having a retardation in a thickness direction thereof; and an optical compensation layer disposed between the first polarizing film and the first substrate, wherein: the optical compensation layer has a birefringence satisfying the relationship 0.0≦(ns−nz)/(ns−nf)≦0.5, where ns, nf and nz are a refractive index along an in-plane slow axis, a refractive index along an in-plane fast axis, and a refractive index along a thickness direction, respectively, of the optical compensation layer; an angle between the in-plane slow axis of the optical compensation layer and the first optical axis is within ±2 degrees; and the optical compensation layer has an in-plane retardation N1 (nm), and the second protective layer of the second polarizing film has a retardation Rt2:
      R    t2    =            (                                                  npx              2                        +                          npy              2                                2                -                  npz          2                    )        ×          d      2      in the thickness direction thereof, where npx2, npy2, npz2 and d2 are a refractive index along an in-plane slow axis, a refractive index along an in-plane fast axis, a refractive index along an orthogonal axis and a thickness, respectively, of the second protective layer of the second polarizing film; and the N1 and Rt2 satisfy therebetween the following relationship:83.050−1.18Rt2≦N1≦228.090−1.08Rt2.in a range of 0≦Rt2≦55 nm.
The present invention also provides, in a sixth aspect thereof, a polarizing film pair for use in a liquid crystal display (LCD) device, including: a first polarizing film including a first protective layer, a first polarization layer having a first absorption axis, a first retardation film having an optical axis normal to a surface of the first polarization layer, the first retardation film having a negative-single-axis birefringence having an in-plane retardation in a range of 0 to 15 nm and an orthogonal retardation of 50 to 123 nm, and a second retardation film having an optical axis parallel to the surface of the polarization layer and the first absorption axis, the second retardation film having a negative uniaxial birefringence having an in-plane retardation of 83 to 210 nm, the second retardation film having different refractive indexes (no) and (ne) along in-plane optical axes extending parallel to each other, and a refractive index (nz) along a direction normal to the surface of the polarization layer, the refractive indexes satisfying the relationship no=nz>ne, the first protective layer, the first polarization layer, the first retardation film and the second retardation film being consecutively layered; and a second polarizing film including a second protective layer, a second polarization layer having a second absorption axis, and a third retardation film having a birefringence having an in-plane retardation of 0 to 10 nm and an orthogonal retardation of 0 to 35 nm, the second protective layer, the second polarization layer and the third retardation layer being consecutively layered.
The present invention also provides, in a seventh aspect thereof, an LCD device including a liquid crystal (LC) layer having a homogeneous orientation, a pair of substrates sandwiching therebetween the LC layer, the polarizing film pair of the present invention sandwiching therebetween the substrates and the LC layer.
In accordance with the IPS LCD device of the present invention, the leakage light leaking upon display of black in the slanted viewing direction can be reduced to improve the contrast ratio of the LCD device, and the chromaticity shift between the normal viewing direction and a slanted viewing direction can be suppressed to thereby improve the image quality of the IPS LCD device.
The above and other objects, features and advantages of the present invention will be more apparent from the following description, referring to the accompanying drawings.